1. Field of the Invention
The present invention relates to processing of semiconductor wafers such as slices of semiconductor silicon and, more particularly, to an improved method and apparatus for polishing the wafers in the chemical-mechanical planarization process in order to achieve a high degree of wafer planarity.
2. Problem to be Solved
In the manufacture of integrated circuits, wafer surface planarity is of extreme importance. Photolithographic processes are typically pushed close to the limit of resolution and it is essential that the wafer surface be highly planar so that the electromagnetic or other radiation used to create the integrated circuit may be accurately focused in a single level thus resulting in precise imaging over the entire surface of the wafer. Wavy, curved or wedge-shaped semiconductor disks result in lack of definition when, for example, a photosensitive resist is applied to the surface of the disk and exposed.
In order to achieve the degree of planarity required to produce ultra high density integrated circuits, chemical-mechanical planarization processes are now typically employed in the industry. In general, the chemical-mechanical planariztion (CMP) process involves pressing a semiconductor wafer against a moving polishing surface that is wetted with a chemically reactive, abrasive slurry. Slurries are usually either basic or acidic and generally contain alumina or silica particles. The polishing surface is typically a planar pad made of a relatively soft, porous material such as blown polyurethane. The pad is usually mounted on a planar platen.
In general, the wafer is secured to a carrier plate (or wafer carrier) by vacuum or by a mounting medium such as an adhesive, with the wafer having a force load applied thereto through the carrier by a pressure plate so as to press the wafer into frictional contact with a polishing pad mounted on a rotating turntable. The carrier and pressure plate also rotate as the result of either the driving friction from the turntable or rotation drive means directly attached to the pressure plate. In a typical polishing machine, the movement of the carrier is programmed to acquire a wafer from a first station, to transport the wafer to a polishing surface, to drive the wafer across the rotating polishing surface, to transport the wafer from the polishing surface to a second station, and to release the wafer at the second station. A typical way of securing and releasing the wafer is by the use of a vacuum head that includes a rigid perforated plate against which the wafer is drawn by applying a vacuum to a plenum lying above the perforated plate.
During polishing it has been found that when a load is imposed on the wafer the polishing action across the wafer is not uniform causing center-to-edge non uniformity in thickness and poor flatness of the wafer. The surface life of the polishing cloth is also a factor in effecting the planarity of the polished wafer. Frictional heat generated at the wafer surface enhances the chemical action of the polishing fluid and thus increases the polishing rate. The frictional head however can cause planarity problems unless the heat is evenly transmitted over the surface of the wafer and typical polishing systems utilize cooling systems to control the temperature of the polishing operation.
A number of attempts have been made in the prior art to improve the planarity of CMP operations. In U.S. Pat. No. 4,270,316 the uneven transmission of pressure which causes different degrees of abrasion of the polished disks is compensated for by the provision of soft elastic inserts placed between a pressure piston and the back of the carrier plate on which the disks to be polished are cemented. In U.S. Pat. No. 4,313,284 a deformable thin disk carrier is mounted through a resilient device to a rotatable pressure plate so that the carrier can be deformed to either a concave shape or convex shape depending on the polishing required. In U.S. Pat. No. 4,910,155 a dam is provided on the polishing plate so that the polishing pool of slurry completely immerses the polishing pad. In U.S. Pat. No. 4,918,869 the use of pressurized air acting on the pressure plate is provided so that the pressure on the wafer surface can be uniform. In U.S. Pat. No. 5,036,630 the wafer carrier comprises at least two (2) materials having different coefficients of thermal expansion which carrier imparts a desired convex or concave bias to the wafer during the polishing operation. In U.S. Pat. No. 5,423,716 the lower face of the backing plate of the wafer carrier includes a number of recessed areas to which a vacuum can selectively be applied. The vacuum is applied to suck a resilient membrane into the recessed areas to draw the wafer into position. The same apparatus can be used to apply a pressurized fluid to the wafer to exert a uniform downward pressure on the wafer. In U.S. Pat. No. 5,486,129 the pressure head of the wafer carrier contains a number of pressure applicators over the wafer surface which can be monitored and adjusted to vary the pressure on the wafer during the polishing operation.
In U.S. Pat. No. 5,232,875 a CMP apparatus is shown wherein the polishing pad is moved about in a plane parallel to the pad surface with either an orbital, fixed-direction vibratory or random-direction vibratory motion.
The disclosures of the above patents are hereby incorporated by reference.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an apparatus, e. g., CMP apparatus, for polishing semiconductor wafers and other workpieces.
It is another object of the present invention to provide an improved method for polishing workpieces, e. g., wafers, using such polishing operations as the CMP system.
It is an additional object of the invention to provide wafer carriers for use in CMP apparatus and CMP processes to enhance operation of the apparatus and improve the flatness of a polished wafer.
It is further object of the invention to provide flat workpieces, including semiconductor wafers, made using the improved method and apparatus of the invention.
Other objects and advantages of the present invention will be readily apparent from the following description.